First-principles study of transport of V doped boron nitride nanotube

We perform first-principles calculations of spin-dependent quantum transport in V doped boron nitride nanotube: the junction of pristine (6,0) boron nitride nanotube in contact with V doped (6,0) boron nitride nanotube electrodes. Large tunnel magnetoresistance and perfect spin filtration effect are obtained. The zero bias tunnel magnetoresistance is found to be several thousand percent, it reduces monotonically to zero with a voltage scale of about 0.65 V, and eventually goes to negative values after the bias of 0.65 V. The ratio of spin injection is above 95% till the bias of 0.85 V and is even as large as 99% for the bias from 0.25 eV to 0.55 eV when the magnetic configurations of two electrodes are parallel. The understanding of the spin-dependent nonequilibrium transport is presented by investigating microscopic details of the transmission coefficients.     

Flat layered structure and improved photoluminescence emission from porous silicon microcavities formed by pulsed anodic etching

Single-mode, highly directional and stable photoluminescence (PL) emission has been achieved from porous silicon microcavities (PSMs) fabricated by pulsed electrochemical etching. The full width at half maximum (FWHM) of the narrow PL peak available from a freshly etched PSM is about 9 nm. The emission concentrates in a cone of 10° around the normal of the sample, with a further reduced FWHM of ∼5.6 nm under angle-resolved measurements. Only the resonant peak is present in such angle-resolved PL spectra. No peak broadening is found upon exposure of the freshly prepared PSM to a He-Cd laser beam, and the peak becomes somewhat narrower (∼5.4 nm) after the PSM has been stored in an ambient environment for two weeks. At optimized etching parameters, even a 4-nm FWHM is achievable for the freshly etched PSM. In addition, scanning electron microscopy (SEM) plane-view images reveal that the single layer porous Si formed by pulsed current etching is more uniform and flatter than that formed by direct current (dc) etching, demonstrated by the well-distributed circular pores with small size in the former in comparison with the irregular interlinking pores in the latter. The SEM cross-section images show the existence of oriented Si columns of 10 nm diameter along the etching direction within the active layer, good reproducibility and flat interfaces. It is thus concluded that pulsed current etching is superior to dc etching in obtaining flat interfaces within the distributed Bragg reflectors because of its minor lateral etching. Received: 7 March 2001 / Accepted: 23 July 2001 / Published online: 30 October 2001     

Surface characterization of self-assembled N-Cu nanostructures

We report on the process of low energy N₂⁺ implantation and annealing of a Cu(0 0 1) surface. Through AES we study the N diffusion process as a function of the substrate temperature. With STM and LEIS we characterize the surface morphology and the electronic structure is analyzed with ARUPS. Under annealing (500 < T < 700 K) N migrates to the surface and reacts forming a Cu_xN compound that decomposes at temperatures above 700 K. LEIS measurements show that N locates on the four-fold hollow sites of the Cu(0 0 1) surface in a c(2 × 2) arrangement. Finally, a gap along the [0 0 1] azimuthal direction is determined by ARUPS. DFT calculations provide support to our conclusions.     

Structural and elastic properties of Zr<Subscript>2</Subscript>AlX and Ti<Subscript>2</Subscript>AlX (X = C and N) under pressure effect

Using first-principles density functional calculations, the effect of high pressures, up to 20 GPa, on the structural and elastic properties of Zr₂AlX and Ti₂AlX, with X = C and N, were studied by means of the pseudo-potential plane-waves method. Calculations were performed within the local density approximation to the exchange-correlation approximation energy. The lattice constants and the internal parameters are in agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline Zr₂AlX and Ti₂AlX aggregates. We estimated the Debye temperature of Zr₂AlX and Ti₂AlX from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Zr₂AlC, Zr₂AlN and Ti₂AlN compounds, and it still awaits experimental confirmation.     

First principles study of structural,magnetic and electronic properties of half-metallic CrO<Subscript>2</Subscript> under pressure

A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method within the local-density approximation is used to calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic CrO₂ at ambient as well as at high pressure. The magnetic and structural stabilities are determined from the total energy calculations. From the present study we predict a magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM) state at 65 GPa, which is of second order in nature. We also observe from our calculations that CrO₂ is more stable in tetragonal phase (rutile-type) at ambient conditions and undergoes a transition to an orthorhombic structure (CaCl₂-type) at 9.6 GPa, which is in good agreement with the experimental results. We predict a second structural phase transition from CaCl₂- to fluorite-type structure at 89.6 GPa with a volume collapse of 7.3%, which is yet to be confirmed experimentally. Interestingly, CrO₂ shows half metallicity under ambient conditions. After the first structural phase transition from tetragonal to orthorhombic, half metallicity has been retained in CrO₂ and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a pressure of 65 GPa.     

Electric field gradient at Ru nuclei in the series of intermetallic compounds Ce1−xLaxRu2 synthesized at high pressure

Parameters of the electric quadrupole interaction for the first excited state (E=89.7 keV) of ⁹⁹Ru nuclei for a number of the cubic Laves phase compounds Ce_1−xLa_xRu₂, synthesized at high pressure, were determined by the perturbed angular γγ-correlation method. Compounds were synthesized at 8 GPa. It was revealed that the decrease of the average valence of a rare earth ion, caused by the substitution of La for Ce, results in the monotonous decrease of the quadrupole frequency ν_Q from 43.3 MHz for CeRu₂ to 33.1 MHz for LaRu₂.     

First-principle calculation on the structural stability of CeAg

The structural stability of CeAg has been studied by self-consistent full-potential linearized augmented plane wave method (FP_LAPW) based on the density functional theory (DFT). The result shows that the low-temperature phase of CeAg is not a simple tetragonal structure. The degenerate d states at the Fermi level are split because of atomic shifts, which result in the cubic-to-tetragonal transition.     

Geometry and electronic properties of single vacancies in achiral carbon nanotubes

An ideal single vacancy can be formed by removing one carbon atom from a hexagonal network. The vacancy is one of the most important defect structures in carbon nanotubes (CNTs). Vacancies can affect the mechanical, chemical, and electronic properties of CNTs. We have systematically investigated single vacancies and their related point defects for achiral, single-walled carbon nanotubes (SWNTs) using first-principles calculations. The structures around single vacancies undergo reconstruction without constraint, forming ground-stateor metastable-state structures. The 5-1DB and 3DB point defects can be formed in armchair CNTS, while the 5-1DB-P and 5-1DB-T point defects can be formed in zigzag CNTs. The related point defects can transform into each other under certain conditions. The formation energies of armchair CNTs change smoothly with the tube radius, while in the case of the 3DB defect, as the radius get larger, the formation energies tend towards a constant value.     

Influence of hydrogenation on the structural and magnetic properties of compounds based on cerium and crystallizing in the tetragonal CeFeSi-type structure

The hydrogen absorption properties of CeMnGe, CeFeSi and CeCoX (X=Si and Ge) have been investigated. Neutron powder diffraction performed on deuteride CeCoGeD indicates that D-atoms are inserted in the pseudo-tetrahedral interstices [Ce₄] of the tetragonal CeFeSi-type structure of this compound. Magnetization and electrical resistivity measurements reveal that the hydrogenation of: (i) CeCoSi and CeCoGe leads to the transition antiferromagnet→spin fluctuation behaviour; (ii) CeMnGe suppress the magnetic ordering of the Ce-moments. These results which suggest a lost of ordered magnetic moment on the Ce site after hydrogenation could result from the chemical effect of hydrogen which prevails over the unit cell expansion effect.     

Strain induced ferroelectricity in the SrZrO3 / SrTiO3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO₃/ SrTiO₃ superlattice is studied using first principles density functional theory. Within the tetragonal symmetry , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO₃ nor SrTiO₃ is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO₃ cell may make a greater contribution to the polarization in the SrZrO₃/ SrTiO₃ superlattice than the SrTiO₃ cell. An extremely large polarization of 42.7 μC/cm² has been predicted.